Pluto’s global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data

Abstract

On July 14th 2015, NASA's New Horizons mission gave us an unprecedented detailed view of the Pluto system. The complex compositional diversity of Pluto's encounter hemisphere was revealed by the Ralph/LEISA infrared spectrometer on board of New Horizons. We present compositional maps of Pluto defining the spatial distribution of the abundance and textural properties of the volatiles methane and nitrogen ices and non-volatiles water ice and tholin. These results are obtained by applying a pixel-by-pixel Hapke radiative transfer model to the LEISA scans. Our analysis focuses mainly on the large scale latitudinal variations of methane and nitrogen ices and aims at setting observational constraints to volatile transport models. Specifically, we find three latitudinal bands: the first, enriched in methane, extends from the pole to 55 • N, the second dominated by nitrogen, continues south to 35 • N, and the third, composed again mainly of methane, reaches 20 • N. We demonstrate that the distribution of volatiles across these surface units can be explained by differences in insolation over the past few decades. The latitudinal pattern is broken by Sputnik Planitia, a large reservoir of volatiles, with nitrogen playing the most important role. The physical properties of methane and nitrogen in this region are suggestive of the presence of a cold trap or possible volatile stratification. Furthermore our modeling results point to a possible sublimation transport of nitrogen from the northwest edge of Sputnik Planitia toward the south.

The Global Color of Pluto from New Horizons

“…The concentric region around the center of Cthulhu Regio has a location along this mixing line abutting the most red terrain (Figure 9). The color of the red terrain is consistent with the presence of tholins as inferred from infrared spectroscopy of Pluto from New Horizons (Protopapa et al 2017). While tholins can exhibit different colors depending on the initial composition and the irradiation history, they are typically a dark red.…”

“…This yellow color unit is generally associated with higher elevation terrain (discussed more below). This color unit is correlated not only with high elevations, but also with a high fractional abundance of CH 4 :N 2 and a reduced fractional abundance of N 2 :CH 4 ( 20% < ) as shown in Figure 13 (Protopapa et al 2017). The solid solution of CH 4 and N 2 can exist in a CH 4 -rich state (CH 4 :N 2 ) or in a N 2 -rich state (N 2 :CH 4 ; Protopapa et al 2015).…”

Volatile evolution and atmospheres of Trans-Neptunian objects

“…The N 2 ice on Pluto is concentrated in a large, deep, N 2 -filled basin called Sputnik Planitia, near the equator and opposite Charon, in which some CO and CH 4 is diluted in solid solution with large-grained or annealed β-N 2 . N 2 -rich ice of similar composition is also seen at mid latitudes, 35-55° N (Protopapa et al 2017;Schmitt et al 2017). It is now understood that Pluto has more CH 4 than Triton overall.…”

Pluto's Beating Heart Regulates the Atmospheric Circulation: Results From High‐Resolution and Multiyear Numerical Climate Simulations

“…The GCM simulations presented in this paper are derived from our most realistic VTM simulations, which best reproduce the threefold increase of surface pressure between 1988 and 2015 (Meza et al, ), with 1–1.2 Pa in 2015 (Stern et al, ). We found three possible scenarios for the surface ice distribution: Scenario #1: No ice deposits outside Sputnik Planitia. Scenario #2: ice deposits in the low‐elevated terrains of the northern midlatitudes, as observed by New Horizons (Protopapa et al, ; Schmitt et al, ). Scenario #3: Same as Scenario #2 but with extra ice deposits in the nonobserved southern hemisphere. …”

“…• Scenario #1: No N 2 ice deposits outside Sputnik Planitia. • Scenario #2: N 2 ice deposits in the low-elevated terrains of the northern midlatitudes, as observed by New Horizons (Protopapa et al, 2017;Schmitt et al, 2017). • Scenario #3: Same as Scenario #2 but with extra N 2 ice deposits in the nonobserved southern hemisphere.…”